1. Field of the Invention
The present invention relates generally to a mobile communications system. In particular, the present invention relates to a system and method of enabling a User Equipment (UE) to reselect to a cell accurately and efficiently.
2. Description of the Related Art
Today's mobile communications systems as being developed, are moving toward high-speed, high-quality wireless data packet communications systems for providing data service and multimedia service beyond the traditional voice-oriented service. Particularly, Universal Mobile Telecommunication Service (UMTS), a 3rd generation mobile communication system using Wideband Code Division Multiple Access (WCDMA) based on the European Global System for Mobile communications (GSM) system, provides mobile subscribers or computer users with a uniform service of transmitting packet-based text, digitized voice, and video and multimedia data at or above 2 Mbps, irrespective of their locations around the world.
The UMTS system includes a Core Network (CN) and a plurality of Radio Network Subsystems (RNSs). The RNSs form a UMTS Terrestrial Radio Access Network (UTRAN). Each RNC has a Radio Network Controller (RNC) and a plurality of Node Bs. The RNCs are classified as a serving RNC (SRNC), a drift RNC (DRNC), and a controlling RNC (CRNC) according to their roles. The SRNC is responsible for management of information about UEs and data transmission to the CN. The DRNC is wirelessly connected to a UE directly. The CRNC controls the radio resources of individual Node Bs.
The RNCs each assign radio resources to a plurality of Node Bs under their control, and the Node Bs provide the assigned radio resources to UEs. The radio resources are configured on a cell basis, and radio resources provided by each Node B are those of a particular cell managed by the Node B. A UE establishes a radio channel with radio resources available to its serving cell and transmits/receives data on the radio channel. The distinction between the Node B and the cell is of little importance to the UE because the UE is aware of only physical channels configured on a cell basis. Therefore, the terms “Node B” and “cell” are interchangeably used herein.
When a UE having no dedicated channels established moves from one cell to another, a cell reselection is triggered. The UE measures the strengths of signals received from neighbor cells, reports the measurements, and finally selects/reselects a cell in a cell reselection procedure. The cell reselection procedure aims to accurately detect the best cell on which the UE will camp and set the best cell as a serving cell for the UE.
For cell reselection, the UE first acquires a neighbor cell list for inter-cell, intra-cell and inter-Radio Access Technology (RAT) cell reselection from a System Information Block (SIB) received from a serving cell on a Broadcast CHannel (BCH). The UE then measures the strengths of signals received from neighbor cells and calculates S_criteria values and R values for cell reselection. The UE selects the best suitable cell according to R value-based cell rankings. The cell reselection criteria are described in greater detail below.
If the UE has camped on the current serving cell for longer than one second and the highest-ranked cell in the neighbor cell list is kept higher in ranking than the current serving cell for a predetermined period of time Treselection, the UE reselects to the highest-ranked cell as a serving cell. The period of time Treselection, which the UE receives in an SIB, is set so as to prevent the UE from reselecting to an instantly higher-ranked cell and thus avoiding a “ping-pong effect”, that is, the repeated handover between cells in a relatively short time.
The period Treselection is different for a high-speed UE and a low-speed UE. The period Treselection set in an SIB is applied to the low-speed UE, whereas the high-speed UE scales down this Treselection value. How much the Treselection is scaled down is determined according to an “intra-frequency Scaling Factor for Treselection”, that is also notified by an SIB. For example, given a Treselection value of 5 s (seconds) and a scaling factor of 0.2, Treselection for the high-speed UE is 1 s (1=5×0.2). In a non-Hierarchical Cell Structure (non-HCS), if the UE changes its serving cell more than nonHCS- Ncr times for a predetermined period nonHCS- Tcrmax, it considers itself a high-speed UE. The nonHCS- Tcrmax and nonHCS- Ncr are also indicated by an SIB.
However, in the case where the UE enters a handover region where cell areas are overlapped and becomes stationary or moves slowly, the number of cell changes may increase due to frequent cell reselections in the handover region. Thus, the UE uses a scaled-down Treselection in the high-speed UE state. As the UE repeats cell reselection for each scaled-down Treselection, the ping-pong effect becomes more serious.
Accordingly, a need exists for a system and method of enabling a User Equipment (UE) to reselect to a cell more accurately and efficiently.